A preferred method for oxidative fracturing permeability enhancement oxidant for shale reservoirs

Abstract

The present application relates to a kind of shale reservoir oxidation fracturing permeability increasing oxidant preferred method.Oxidant type of shale reservoir oxidation fracturing permeability increasing reconstruction is numerous, for example, oxidant hydrogen peroxide, sodium persulfate, ammonium persulfate, sodium hypochlorite, potassium permanganate and other oxidants can efficiently enhance the percolation capacity of shale reservoir.Different oxidants have obvious advantages and disadvantages, therefore, for specific shale reservoir, the preferred suitable oxidant has important significance for giving full play to the effect of oxidation fracturing permeability increasing.The present application establishes a kind of shale reservoir oxidation fracturing permeability increasing oxidant preferred method, the oxidation effect of different oxidants can be quantified, and the preferred result is accurate, which provides basis for well selection and layer selection of shale reservoir oxidation fracturing permeability increasing reconstruction.

Description

Technical Field

[0001] This invention belongs to the field of fracturing fluid technology for shale oil and gas reservoirs, and relates to a method for selecting the optimal oxidant for oxidative fracturing and permeability enhancement in shale reservoirs. It is mainly used to select the optimal oxidant suitable for oxidative fracturing stimulation of shale reservoirs. Background Technology

[0002] Shale reservoirs are generally characterized by low porosity and low permeability, resulting in poor natural oil and gas flow capacity and a lack of natural production capacity. Therefore, a complex fracture network formed after hydraulic fracturing is needed to establish high-speed flow channels. Considering that shale is rich in reducing minerals such as pyrite, chlorite, and organic matter, invention patent CN201510821577.3, "A Method for Improving the Permeability of Organic-Rich Shale Blocks," discloses a method that utilizes an oxidant to oxidize and decompose reducing minerals such as pyrite, chlorite, and organic matter within shale, forming dissolution pores, expanding gas flow channels in the shale block, increasing the connectivity of flow channels, and thus improving shale permeability.

[0003] Numerous types of oxidants are used for oxidative fracturing and permeability enhancement in shale reservoirs. Oxidants such as hydrogen peroxide, sodium persulfate, ammonium persulfate, sodium hypochlorite, and potassium permanganate can all effectively enhance the permeability of shale reservoirs. The advantages and disadvantages of different oxidants are very obvious. For example, sodium hypochlorite only has a good dissolving effect on pyrite and organic matter, but it has no dissolving effect on carbonate minerals and may produce ferric hydroxide precipitation. Therefore, selecting the appropriate oxidant for a specific shale reservoir is of great significance for fully realizing the effects of oxidative fracturing and permeability enhancement. Summary of the Invention

[0004] The purpose of this invention is to provide a preferred method for using an oxidizing fracturing and permeability-enhancing oxidant in shale reservoirs, characterized by the following steps:

[0005] S1: Based on the geological characteristics of shale reservoirs, three oxidants suitable for oxidative fracturing and permeability enhancement were initially selected and named oxidant a, oxidant b, and oxidant c, respectively.

[0006] S2: Construct the hierarchical structure of the analytic hierarchy process, which consists of the target layer: selecting the optimal oxidant for a certain shale block (Z); the index layer: organic matter volume content (A1), pyrite volume content (A2), chlorite volume content (A3), calcite volume content (A4), dolomite volume content (A5); and the scheme layer: oxidant a (B1), oxidant b (B2), and oxidant c (B3).

[0007] S3: Test the mineral composition of representative reservoir rock samples to obtain the contents of organic matter, pyrite, chlorite, calcite, and dolomite;

[0008] S4: Calculate the volume of organic matter, pyrite, chlorite, calcite, and dolomite based on the mineral density;

[0009] S5: Construct a comparative discrimination matrix Z based on the volume content of organic matter, pyrite, chlorite, calcite, and dolomite. (A) =(a ij )5,a ij This indicates the relative importance of the i-th factor to the j-th factor (a). ij = 1, 2, ..., 9 and their reciprocals, where 1 indicates that factor i and factor j are equally important, and 9 indicates that factor i is extremely more important than factor j;

[0010] S6: Conduct dissolution experiments on shale with oxidants a, b, and c, and calculate the dissolution volume of organic matter, pyrite, chlorite, calcite, and dolomite;

[0011] S7: Discriminant matrix A: Based on the comparative analysis of the volumetric structures of organic matter, pyrite, chlorite, calcite, and dolomite by different oxidants (hydrogen peroxide, persulfate, and hypochlorite). 1(B) =(a ij 3. A 2(B) =(a ij 3. A 3(B) =(a ij )3, A 4(B)) =(a ij )3, A 5(B) =(a ij )3, corresponding to the comparison and discrimination matrices of organic matter, pyrite, chlorite, calcite, dolomite and oxidant respectively;

[0012] S8: Calculate matrix Z (A) A 1(B) A 2(B) A 3(B) A 4(B) A 5(B) The largest eigenvalue λ max The consistency index CR is calculated, and the reasonableness of the matrix is ​​judged based on the range of the consistency ratio. When CR < 0.1, the matrix is ​​considered to be reasonably consistent; when CR ≥ 0.1, it needs to be corrected. The specific calculation method is as follows:

[0013]

[0014] Where RI is the average random consistency index, and n is the matrix dimension. When n = 3, RI = 0.58; when n = 5, RI = 1.12.

[0015] S9: For matrix Z (A) A 1(B) A 2(B) A3(B) A 4(B) A 5(B) The algorithm is calculated layer by layer from top to bottom to obtain the ranking weight vector W = (b1, b2, b3) of the algorithm layer relative to the overall goal. When b1 is the largest, the optimal oxidant is oxidant a (B1); when b2 is the largest, the optimal oxidant is oxidant b (B2); when b3 is the largest, the optimal oxidant is oxidant c (B3).

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] (1) This method can quantify the oxidation effect of different oxidants, and the selection of oxidants for fracturing and permeability enhancement in shale reservoirs is based on evidence.

[0018] (2) The method yields accurate results and can provide a basis for well and layer selection for oxidation-induced fracturing and permeability enhancement of shale reservoirs. Attached Figure Description

[0019] Figure 1 This is the hierarchical structure of the analytic hierarchy process used in this invention. Detailed Implementation

[0020] To provide a clearer understanding of the technical features, objectives, and beneficial effects of this invention, the technical solution of this invention will be described in detail below with reference to embodiments. These embodiments are only used to further illustrate this invention and should not be construed as limiting the scope of protection of this invention. Non-essential improvements and adjustments made by those skilled in the art based on the content of this invention also fall within the scope of protection of this invention.

[0021] Based on the method of the present invention, the following specific embodiment is given to verify the technical solution of the method of the present invention in the organic-rich shale reservoir of Block A:

[0022] (1) Based on the geological characteristics of the organic-rich shale reservoir in Block A, three oxidants suitable for oxidative fracturing and permeability enhancement were initially selected: hydrogen peroxide, sodium persulfate, and sodium hypochlorite.

[0023] (2) Whole-rock mineral and TOC content tests showed that the organic matter, pyrite, chlorite, calcite, and dolomite contents of the organic-rich shale samples in Block A were 4.2%, 3.0%, 6.6%, 11.9%, and 5.7%, respectively, with densities of 1.25 g / cm³. 3 4.90 g / cm 3 2.70 g / cm 3 2.71 g / cm 3 2.86 g / cm 3The calculated volume percentages are 3.36%, 0.61%, 2.44%, 2.19%, and 1.00%, respectively. Therefore, the construction comparison matrix of A relative to Z is:

[0024]

[0025] (3) Based on the whole-rock mineral and TOC content test results after shale dissolution experiments with oxidants hydrogen peroxide, sodium persulfate, and sodium hypochlorite, the comparison discriminant matrices of B relative to A are as follows:

[0026]

[0027] (4) Compare the discriminant matrix Z (A) A 1(B) A 2(B) A 3(B) A 4(B) A 5(B) The largest eigenvalue λ max The values ​​are 5.0416, 3.0055, 3.0183, 3.0536, 3.1083, and 3.1083, respectively, and the CR values ​​are 0.0093, 0.0047, 0.0158, 0.0462, 0.0158, and 0.0158, respectively. All of these values ​​are less than 0.1, which meets the consistency requirements and requires no correction.

[0028] (5) For matrix Z (A) A 1(B) A 2(B) A 3(B) A 4(B) A 5(B) By calculating layer by layer from top to bottom, the ranking weight vector W = (0.2688, 0.40956, 0.32156) of the scheme layer relative to the overall objective is obtained. The maximum value is 0.40956, therefore, sodium persulfate is selected as the optimal oxidant for the organic-rich shale reservoir in Block A.

Claims

1. A preferred method for oxidative fracturing permeability enhancing oxidizing agent for shale reservoirs characterized by Comprise the following steps: S1: According to the shale reservoir geological characteristics, initially select three kinds of suitable for oxidation cracking permeability increasing oxidants, respectively named as oxidant a, oxidant b, oxidant c; S2: Construct the hierarchical structure of analytic hierarchy process, respectively for target layer: select the optimal oxidant (Z) of certain shale block, index layer: organic matter volume content (A1), pyrite volume content (A2), chlorite volume content (A3), calcite volume content (A4), dolomite volume content (A5), scheme layer: oxidant a (B1), oxidant b (B2), oxidant c (B3); S3: Test the mineral components of reservoir representative rock sample, obtain the contents of organic matter, pyrite, chlorite, calcite and dolomite; S4: According to the mineral density, calculate the volumes of organic matter, pyrite, chlorite, calcite and dolomite; S5: Discriminant matrix Z is constructed by comparing the volume content of organic matter, pyrite, chlorite, calcite and dolomite (A) = (a ij )5, a ij represents the relative importance of the ith factor and the jth factor, a ij = 1, 2…, 9 and its reciprocal, where 1 indicates that factor i and factor j are equally important, and 9 indicates that factor i is extremely important than factor j; S6: Carry out the dissolution experiment of shale and oxidant a, oxidant b, oxidant c, and calculate the dissolution volumes of organic matter, pyrite, chlorite, calcite and dolomite; S7: Comparison discriminant matrix A of dissolution volume structure of organic matter, pyrite, chlorite, calcite and dolomite according to different oxidants hydrogen peroxide, persulfate and hypochlorite 1(B) = (a ij )3, A 2(B) = (a ij )3, A 3(B) = (a ij )3, A 4(B)) = (a ij )3, A 5(B) = (a ij )3, respectively corresponding to comparison discriminant matrix of organic matter, pyrite, chlorite, calcite and dolomite and oxidant; S8: Calculate the matrix Z (A) , A 1(B) , A 2(B) , A 3(B) , A 4(B) , A 5(B) The maximum eigenvalue λ max of the matrix Z, calculate the consistency index CR, and determine whether the matrix is reasonable according to the range to which the consistency ratio belongs. When CR<0.1, it is determined that the matrix consistency is reasonable; when CR≥0.1, it needs to be modified, and the specific calculation method is as follows: Wherein, RI is the average random consistency index, n is the matrix dimension, when n=3, RI=0.58; when n=5, RI=1.12; S9: To matrix Z (A) , A 1(B) , A 2(B) , A 3(B) , A 4(B) , A 5(B) From top to bottom, layer by layer, the sorting weight vector W = (b1, b2, b3) of the total target is obtained, when b1 is the largest, the optimal oxidant is oxidant a (B1); when b2 is the largest, the optimal oxidant is oxidant b (B2); when b3 is the largest, the optimal oxidant is oxidant c (B3).

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